The invention relates to a method for the preparation of highly monodisperse, nonporous spherical SiO.sub.2 particles and also to the SiO.sub.2 particles produced by the method.
Spherical SiO.sub.2 particles are of special interest as valuable aids in the technical and scientific field and are also the object of scientific study. An important field of application of SiO.sub.2 particles, especially accurately defined particles of uniform size, principally in the nm and .mu.m region, is their use in standardization. For example, SiO.sub.2 particles are used as calibration standards for determining the size of small objects such as dust particles or cells. A further field of application for these particles is their use as sorption or carrier materials in the field of chromatography and in separation techniques derived therefrom. In all such applications, particle size and particle size distribution play a considerable part. Therefore, it is important to be able to produce such particles in a predeterminable nd reproducible manner in relation a to particle size characteristics.
That spherical SiO.sub.2 particles can be obtained by hydrolytic polycondensation of tetraalkoxysilanes is known from the prior art, for example, from the publications by W. STOBER et al. in J. Colloid and Interface Science 26, 62 (1968) and 30, 568 (1969) and from U.S. Pat. No. 3,634,588, which disclosures are entirely incorporated by reference herein. From these documents the basic reaction conditions for this purpose can be inferred. These documents disclose a process in which tetraalkoxysilane is introduced into an excess of an aqueous/alcoholic ammoniacal hydrolysis mixture wherein thorough mixing is provided by suitable measures such as stirring, shaking or ultrasonic treatment. In this process, depending on the choice of the specific experimental parameters, SiO.sub.2 particles of various mean particle size and varying particle size distribution can be obtained. According to the data of the publications cited, SiO.sub.2 particles with mean particle sizes between 0.05 and 2 .mu.m (in isolated cases up to approximately 3 .mu.m) were obtained.
Also, the influence of various esters of silicic acid, of ammonia and water concentration and of various alcohols in the hydrolysis mixture were investigated. From the results, which were possible to confirm by in-house investigations, it may be inferred that it is possible to some extent to obtain monodisperse spherical particles only in the particle size region up to about 2 .mu.m, without it as yet being possible, however, to control the reproducibility adequately. Thus, the standard deviations for the particle diameters usually lie between about 5 and 15%. In isolated cases standard deviations of up to 50% were observed. Attempts to prepare monodisperse particles of larger diameter were unsuccessful. The preparation of particles with diameters of over 3 .mu.m was not described. According to the publications cited, the particles were prepared and only characterized in the form of their hydrosoles. The particles were not isolated as particles per se. Consequently, any data on the other properties of the particles, particularly their porosity, are lacking.
In in-house investigations on SiO.sub.2 particles prepared by the method of STOBER et al. and then isolated by sedimentation or centrifugation and subsequent drying, it was found that such particles have a marked microporosity. This manifests itself in the specific surface area, which can be measured, for example, by gas adsorption (for instance, by the BET method) and which, depending on the given experimental conditions, exceeds the surface area calculated theoretically by a factor of 10-100.
Naturally, the microporosity of the particles affects their properties substantially. However, for many of the above mentioned uses of SiO.sub.2 particles, it is regarded as advantageous for the particles to have virtually no porosity, i.e., have a completely closed surface.